diff --git a/user_manual/Makefile b/user_manual/Makefile
index 0976b056d67..c050a87fba0 100644
--- a/user_manual/Makefile
+++ b/user_manual/Makefile
@@ -69,8 +69,8 @@ merge-folders:
 	cd $(BUILDDIR)/html/ && rm -rf `find -type d -name _images`
 	mv ./_static $(BUILDDIR)/html/
 	mv ./_images $(BUILDDIR)/html/
-	cd $(BUILDDIR)/html/ && find ./ -type f -exec sed -i -e 's/_images/..\/_images/g' {} \;
-	cd $(BUILDDIR)/html/ && find ./ -type f -exec sed -i -e 's/_static/..\/_static/g' {} \;
+	cd $(BUILDDIR)/html/ && find ./ -type f -exec perl -pi -e 's|_images|../_images|g' {} \;
+	cd $(BUILDDIR)/html/ && find ./ -type f -exec perl -pi -e 's|_static|../_static|g' {} \;
 
 dirhtml:
 	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml